A conventional so-called overlock sewing machine that carries out overedging forms seams by combining a thread passed through the eye of a needle and a looper thread passed through the looper eye of a lower looper together. Therefore, in order to allow the looper thread to be picked up on a thread guide of the lower looper to be passed through the looper eye, it is necessary to operate a hand pulley until the looper eye and the thread guide of the lower looper are moved to a position adjacent to an opening on the front of the sewing machine and then to pick up the looper thread on the thread guide that has been moved to the position adjacent to the opening to pass it through the looper eye.
However, due to the relationship with respect to the stitch point of a sewing needle, it is impossible to move the lower looper to the position where the lower looper is exposed sufficiently from the opening. Therefore, it is necessary to carry out an operation of threading the lower looper in a small space and thus the operation of threading the lower looper is a troublesome operation, which has been a problem. Furthermore, there is also a possibility that when the lower looper is activated during a threading operation, the threading device for the lower looper may be damaged.
Therefore, for example, Patent Document 1 discloses a safety device for an overlock sewing machine that detects the position of the looper and can prevent the main shaft from rotating while a detection signal indicating that the looper is in an exposed position is being generated. Furthermore, Patent Document 2 discloses a threading device for a looper that prevents a pulley from rotating by inserting a locking arm into a catching part of the pulley so that the threading device is not damaged even when the sewing machine is started during a threading operation. This prevents the lower looper from operating even when the sewing machine is started by mistake. Thus, the threading device for a looper is not damaged by, for example, oscillation of the lower looper.